Eight of the top 10 sickest places in Britain are in south Wales, according to a survey.
Merthyr Tydfil played a key role in the industrial revolution
Merthyr Tydfil is named as the most unhealthy place in the UK, with Rhondda Cynon Taf and Neath Port Talbot coming third and fourth.
Carmarthenshire, Caerphilly, Torfaen and Bridgend also have the highest levels of serious illness and the poorest diets, the report claims.
The researchers warned the results were a "time bomb" for the health service.
The health map, produced by market researchers CACI and TNS, uses a combination of official data, the census and surveys to put local authority areas into four groups.
The second "sick" group - with severely unhealthy lifestyles likely to cause serious illness - is topped by London boroughs Tower Hamlets, Hackney, Southwark and Lambeth.
Researchers named the Isles of Scilly as the healthiest place in the UK - followed by Eden in Cumbria and the Orkney Islands.
Ian Thurman, from CACI, said he was not surprised that south Wales featured so heavily in the most unhealthy group.
Easington, Co Durham
Rhondda Cynon Taf
Neath Port Talbot
Source: HealthAcorn report by CACI and TNS
"It's the classic ex-mining areas, steel works, ship building - all those post-industrial areas," he said.
Mr Thurman warned the results represented a serious problem for the health service.
He added: "The NHS is already overburdened, but this is nothing compared to the time bomb that is set to explode if people don't make major changes to their current lifestyles."
The Welsh Assembly Government said it was encouraging people to take better care of themselves and identified smoking as one of the biggest causes of chronic illness.
A spokeswoman said: "Wales has a history of ill health and chronic conditions due mainly to its industrial past.
"Tackling poor health and health inequalities is a key priority for the assembly government."
The spokeswoman said a number of measures were being introduced to help eradicate the "unacceptable" inequalities, including the introduction of free prescriptions in April and healthy food initiatives in schools.
Kevin Sullivan of the Welsh NHS Confederation - which represents all NHS organisations - said: "Chronic conditions have rightly been called the 21st century healthcare challenge.
"In Wales we have one of the highest levels in the UK, partly due to unhealthy lifestyles. Chronic conditions are also closely linked with age - we have the 3rd highest percentage of over 65s in Europe - combined with the legacy of our industrial past."